Commodity chemicals are typically manufactured through the operation of continuous chemical conversion processes. Continuous conversion technology typically employs the use of continuous flow reactors, which offer certain advantages, such as the ability to prepare large volumes of chemicals (e.g., commodity chemicals) and lower capital and operational expenditures as compared to methods of production that do not employ continuous conversion technology. Continuous flow reactors can be used for a variety of transformations and can be operated in the gas or liquid phase.
5-Hydroxymethylfurfural (“HMF”) is a platform chemical producible from biorenewable resources, particularly carbohydrate-containing feedstocks. The potential of HMF for the production of various compounds useful for fuel, fine chemical, and polymer applications, such as 5-alkoxymethylfurfural, 2,5-furandicarboxylic acid, 5-hydroxymethylfuroic acid, 2,5-bishydroxymethylfuran, 2,5-dimethylfuran, bis(5-methylfurfuryl)ether, levulinic acid, adipic acid, 1,6-hexanediol, caprolactone and caprolactam, has grown with the development of efficient processes for chemically converting HMF on a large-scale (van Putten et al. 2013 Chem Rev 113:1499-1597). However, the purity of HMF derived from a carbohydrate source limits the commercial viability of such processes. HMF is typically prepared from fructose in the presence of a mineral acid (de Vries et al. 2013 Chem Rev 113:1499-1597). This process produces side products such as humins, which are believed to be condensation products from the reaction constituents and can be oligomeric or polymeric in form. Accordingly, HMF feedstock can contain trace amounts of mineral acids and/or trace amounts of oligomeric or polymeric species which may affect the production of HMF conversion products, which are products produced directly or indirectly from the conversion of HMF.
The conversion of HMF to 2,5-bishydroxymethylfuran, 1,6-hexanediol, and other HMF conversion products via reduction using hydrogen and a heterogeneous catalyst has been reported. See, for example, Schiavo et al. 1991 Bull Soc Chim Fr 128:704-711; U.S. Pat. No. 7,994,347; U.S. Pat. No. 8,367,851; U.S. Pat. No. 8,742,144; U.S. Pat. No. 3,070,633; U.S. Pat. No. 3,083,236; U.S. Pat. No. 7,579,490; EP Patent No. 2390247; International Publication No. WO 2011/149339; Buntara et al. 2013 Catal Today 210:106-116; Buntara et al. 2011 Angew Chem Int Ed 50:7083-7087; International Publication No. WO 2013/163540; U.S. Pat. No. 3,040,062, Connolly et al. 2010 Org Process Res Dev 14:459-465, Nakagawa 2010 Catal Commun 12:154-156, International Publication Nos. WO 2014/152366 and WO 2013/109477, and Besson et al. 2014 Chem Rev 114:1827-1870. These processes are typically liquid-phase and, while many produce an HMF conversion product, there remain drawbacks that limit their use. First, batch mode conversions produce limited volumes of product, and commodity chemicals, which are needed in large volumes, cannot be produced as cost effectively using a batch mode. Second, reactions carried out using continuous conversion technology are similarly limited if the reactions employ any of: (i) low feedstock concentrations (if the feed concentration is too low, too much energy and expense will be necessary to recover the target product from the liquid phase); (ii) catalysts that are unstable under the reaction conditions needed for industrial application (such as catalysts that are not stable under many continuous hours on-stream in a continuous flow reactor); or (iii) catalysts that do not have the requisite selectively to produce a sufficient volume of the target HMF conversion product (high selectivity to the desired reaction product is desirable as it minimizes the costs associated with the purification of the product as fewer side products need to be removed). The limitations of current methods demonstrate the need for alternative methods of converting HMF to target HMF conversion products, such as commodity and specialty chemicals, on a commercial scale.